1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus for use in electrophotography.
2. Discussion of the Background
An image forming method including the following steps is known in the field of image formation:
(1) forming an image on an image bearing member by a developing means;
(2) primarily transferring the image from the image bearing member onto an intermediate transfer member by a primary transfer means;
(3) secondarily transferring the image from the intermediate transfer member onto a recording medium by a secondary transfer means; and
(4) fixing the image on the recording medium by a fixing means.
While an image forming method performing each image forming step in a sequential manner is used in the market, published unexamined Japanese patent application Nos. (hereinafter referred to as JP-A) 10-63121 and 2004-145260 have described image forming methods including a process in which the transfer and fixing processes are simultaneously performed. This process will be hereinafter referred to as “transfer-fixing process.” In the former application, an image is transfer-fixed from an intermediate transfer member onto a recording medium. In the latter application, an image is secondarily transfer-fixed from an intermediate transfer member onto a transfer-fixing member, and then thirdly transfer-fixed from the transfer-fixing member onto a recording medium.
A toner, which is a powder constituted of a resin having a chargeability, is typically used as a material for forming an image in the above discussed image forming methods.
In the typical image forming method, the image quality tends to deteriorate when an image is transferred onto a recording medium, which may include papers. Papers may have different thicknesses (e.g., plain paper, thick paper) and different surface natures (e.g., smooth, rough). When a paper having a rough surface is used, an intermediate transfer member cannot faithfully adhere to the surface of the paper, and therefore microgaps are formed on the paper. In the microgaps, an image cannot be normally transferred due to abnormal electrical discharge. As stated above, the typical image forming method has a disadvantage that an abnormal image is easily formed in the transfer process.
On the other hand, the image forming method including the transfer-fixing process has an advantage that the image quality hardly deteriorates, even if a paper having a rough surface is used for the following reasons. In this method, a heat is applied to a toner when transferred, and thereby the toner is softened, melted, and becomes a block having viscoelasticity. The toner block having viscoelasticity can be easily transferred even in an image portion formed on the microgap. It is considered that the image forming method having the transfer-fixing process is a suitable method for producing high quality images.
It is advantageous in terms of energy to heat an image on a transfer member before fixing the image on a recording medium because applying heat only to the image on the transfer member reduces the heat absorbed by the recording medium.
Although the above method has some advantages, the following problems still exist.
(I) When an image having dots is transfer-fixed, latent images of the dots need to be accurately developed. If the dots are developed with a toner having poor dot reproducibility, toner particles tend to be scattered. These scattered toner particles tend not to be transfer-fixed.
(II) When an image on a transfer member is heated, a toner tends to melt and liquefy. Thereby, each of the uniformly formed dots tends to expand, contract, or transform, resulting in poor reproducibility of the latent image. As a result, an image with low image density, blurred, and somber features tends to be formed on the recording medium.